[Ardour-Users] click free editing and zero crossings?

[Lamar Owen]

On Wednesday 19 May 2010 08:49:12 am Jörn Nettingsmeier wrote:
> a) there is nothing magical about zero crossings.

Well, actually there is.  If you're splicing arbitrary clips at arbitrary 
signal levels, zero crossing is the only rough point that all signals (without 
DC bias) will pass through eventually.  But you can just pick a point 
arbitrarily close to zero to make it work.  And as Paul so rightly points out, 
true 'zeros' in a signal are rather rare; you're looking for adjacent positive 
and negative samples to see the zero crossing.  But that makes finding zero 
quite a bit easier than finding any other chosen value;  just look for the sign 
change instead of comparing if two adjacent samples bracket the chosen value.  
So, yes, zero crossing has some magic to it; just not the magic one might 
necessarily think.

> b) you might as well edit at any <arbitrary value> crossing. what helps
> preventing clicks here is the fact that both sides of the splice are at
> roughly the same value. what helps even more is for both splice sides to
> cross <arbitrary value> in the same direction (unlike in my example,
> which was intentionally nasty in that regard).

Ideally, the signal will cross zero and there won't be audible discontinuities 
in the first and second order derivatives of the signal waveform across the 
splice.

> c) splices at similar values (or zero crossings, for that matter) are
> not a 100% guarantee the edit will be clean.

So true; which is one reason Audacity and many other destructive editors don't 
do a sharp cut anyway; there is a very small crossfade involved, at least with 
Audacity and Audition.  Haven't checked Wave Editor or Cubase yet.

But good editing is good editing, and proper crossfades are essential in any 
case.  I'm working on a bluegrass track right now where I'm splicing banjo and 
guitar breaks, and, since the artist doesn't use a click track (says it ruins 
her groove (she says 'timing' but she really means groove); and she's right, 
even in bluegrass, one of the more rhythmically structured genres, the 
'groove' involves the ever so slight 'swaying' of the rhythm; and her timing 
is very good indeed, to where the tightening of timing by using a click track 
would make almost no audible difference), I have to make adjustments....  

So I'm beat-matching the splicing; when done I'll re-lay the bass track rather 
than edit that: since I'm the bassist, I can do that.... :-)  The track is 
quite fast; she did several takes of the break, and she missed a few notes on 
pretty much every take on the guitar part (her banjo part is solid, but 
there's that groove thing again, and when splicing in from the guitar, to get 
an exact beat match (de rigeur for bluegrass is the exact timing of the 
different instruments relative to each other), the banjo has to come with it), 
and I'm putting together a 'best of' montage.  Which is quite fun to do, 
because I'm having to grok her groove and not only beat match but groove match 
(first order derivative of the rhythm.....).  Yes, I'm having a blast doing it. 
Helps my bass playing immensely, too.  

Clicks are not the only issues when editing, especially when editing pad-type 
parts (fiddle, dobro, or harmonica in bluegrass) or electric bass.  If the 
crossfade spans more than a few cycles, you really want the actual cycles 
during the crossfade to phase match; unless you are looking for a phasor effect 
at the splice, that is.  Zooming way in helps this; carefully picking the 
splice points helps more.  Percussive parts, like mandolin and guitar, aren't 
as picky.  Splicing banjo is the most fun, thanks to the percussive attack but 
paddish sustain due to the resonator.  Splicing in the resonator tail is 
easiest, you just have to zoom way in and match cycles across the crossfade.  
A crossfade that spans the attack but is phase matched in the resonator tail 
typically makes the attack sound mushy, since virtually no two banjo notes 
have phase matching between the attack and the resonator tail (most tails lead 
in to the next attack anyway, and really rapid-fire banjo playing never really 
gets out of any single resonator tail.......)

The easiest places I've found to splice vocals is during breaths; inhales 
aren't super picky, and the crossfades aren't critical.